In the background art, construction machines having swing structures such as hydraulic excavators, in each of which a hydraulic pump is driven by an engine and a hydraulic motor is rotated by hydraulic pressure ejected from the hydraulic pump so as to drive the swing structure which is an inertia body, constitute the mainstream. In recent years, however, a hybrid type construction machine in which a swing structure is driven by use of both an electric motor driven by electric energy supplied from an electric storage device and a hydraulic motor has been proposed in order to improve the fuel consumption of an engine, reduce the noise level and reduce the amount of exhaust gas, and so on (for example, see Patent Document 1). In addition to the electric motor used for revolution, another electric motor which is used for generating electric power and which supplies electric energy to the electric storage device when the level of electricity stored in the electric storage device is low is also provided in the construction machine according to Patent Document 1.
In such a hybrid type construction machine, driving torque undertaken by the hydraulic motor and the electric motor is required to be controlled appropriately so that an operator accustomed to operating a background-art construction machine for driving a swing structure by use of only a hydraulic motor can operate without feeling anything wrong. On that occasion, electric energy consumed during driving (motor driving) and generated during braking (regeneration) by the electric motor is stored and released by the electric storage device connected to the electric motor. Since there is a limit in the capacity of the electric storage device, it is particularly important to properly manage the amount of electricity stored in the electric storage device, that is, to perform energy management.
In Patent Document 1, disclosed is a technique in which a torque instruction unit by which an instruction of toque with which the electric motor is driven and controlled is given in connection to a differential pressure between the inlet and the outlet of the hydraulic motor used for swing drive is provided as a control unit for the hybrid type construction machine. In addition, also in Patent Document 1, disclosed is a technique in which the ratio between the torque of the hydraulic motor and the torque of the electric motor during acceleration drive and the ratio between the torque of the hydraulic motor and the torque of the electric motor during deceleration drive are defined using the differential pressure between the inlet and the outlet of the hydraulic motor as a parameter (for example, paragraph 0060). In Patent Document 1, description is made that according to these techniques, the swing structure which is an inertia body can be driven and controlled continuously and smoothly and the energy during braking can be imported as electric energy into the electric storage device effectively (for example, paragraphs 0033 and 0034).